Systematic Review of Algal Toxins in Seals and Sea Lions: Could Unexplained Caspian Seal Mortalities Be Related to Harmful Algae?

Harmful algal blooms (HABs) produce potent neurotoxins, such as domoic acid (DA) and saxitoxin (STX), which negatively impact marine mammal health. To contextualize these threats within specific regional ecosystems, we conducted a comprehensive meta-analysis of algal toxin concentrations in stranded and non-stranded seals and sea lions reported between 1990 and 2025. Following a systematic screening of 2377 records, 15 studies met the criteria for inclusion in our systematic review and quantitative analysis. Both toxins displayed substantial variability among studies. Notably, the concentrations of DA found in feces were markedly higher than in other matrices, and feces appears to be a high-yield matrix for detecting recent exposure. Urine and serum showed moderate DA levels, while fetal fluids were among the lowest. Geographically, most studies originated from California and the Alaskan Arctic. Multi-matrix monitoring of algal toxins is required in remote or logistically challenging regions. We propose that exposure to algal and cyanobacterial neurotoxins is a testable contributor to recurrent, unexplained mass mortality events among Caspian seals (Pusa caspica) and that targeted toxicological screening within the Caspian Sea ecosystem is urgently needed.